The invention relates to electrical discharge machining in which the displacement of a tool-forming electrode (hereinafter called electrode) relative to a workpiece-forming electrode (hereinafter called workpiece) is regulated to maintain a quantity characterizing the sparking conditions at a predetermined value.
When machining a recess.Iadd., cavity.Iaddend. or blind hole in the workpiece, it is known to stop the machining current, usually by means of a micro-switch, as soon as the electrode has penetrated in the workpiece by a predetermined amount. However, precise machining tolerances can only be achieved in this manner by resorting to several tedious operations to take into account the precision of the micro-switch and the presumed spark distance. This distance is difficult to evaluate since it varies with the machining parameters, especially with the conditions of .[.replacement of the.]. .Iadd.renewal of the dielectric or .Iaddend.machining fluid. Moreover, it is not possible to guarantee that the frontal machined surface will be of equal quality to the lateral machined surfaces. Usually, the frontal surface is in .[.a.]. much .[.poorer.]. .Iadd.rougher .Iaddend.state than the lateral ones.
An aim of the invention is to .[.enable.]. .Iadd.eliminate .Iaddend.these disadvantages .[.to be eliminated.]. by providing a specific regulation of the relative position of the electrode and workpiece.
According to the invention, a method of machining by providing intermittent electrical discharges between an electrode and a workpiece while regulating relative displacement of the electrode and workpiece to maintain a quantity characterizing the sparking conditions at a predetermined value, comprises reducing the .[.rate.]. .Iadd.maximum speed .Iaddend.of penetration of the electrode in the workpiece as a function of a quantity representing their relative position in a manner to prevent the electrode from penetrating in the workpiece beyond at least one predetermined final relative position while maintaining the electrical discharges and allowing the electrode and workpiece to be free to move relative to one another as a function of the sparking conditions up to said final relative position.
Hence, when the electrode reaches a predetermined position of penetration, the mean machining current .[.reduces.]. .Iadd.drops .Iaddend.progressively to zero and the machining stops without any mechanical action. An advantage of this method is to progressively reduce the local pollution due to machining residues in the frontal zone which enables, at the end of operation, the obtention of a sparking distance just greater than that necessary for breakdown in a clean dielectric.
In these conditions, the sparking distance has a known, reproduceable value. During the entire phase of stopping machining, the electrodes remain under voltage, and the possibility of rapid withdrawal of the electrode in the case of short-circuiting is maintained.
Another advantage of the new method is to ensure that the state.Iadd., or more specifically the roughness .Iaddend.of the frontal surface is similar to that of the lateral surfaces at the end of each machining operation. .[.This advantage could not be obtained by, for example, a method consisting of applying non-erosive machining conditions with discharges of very low energy, once the electrodes have reached a predetermined relative position..].
Finally, a precise electronic servo-control and knowledge of the distance separating the electrodes at the end of machining enable high precision machining to be carried out, and simplify the operations of setting the dimensions to be machined.